The present invention relates to flexible circuits incorporating electrostatic discharge (ESD) limiting features and designed for use in the fabrication of hard disk drives for computer applications. More specifically, the invention relates to a flexible circuit having a conductive polymer strip which is intended to enhance static dissipative characteristics and to protect the magnetorestistive (MR) heads of a hard disk drive (HDD) from damage during the manufacture of the head gimbal assembly (HGA) of the HDD.
Flexible circuits are circuits that are formed on flexible dielectric substrates including various polymeric materials, e.g., polyimide and the like. The circuits may have one or more conductive layers as well as circuitry on one of the major surfaces or on both major surfaces. The circuits often include additional functional layers, e.g., insulative layers, adhesive layers, encapsulating layers, stiffening layers and the like. Flexible circuits are typically useful for electronic packages where flexibility, weight control and the like are important. In many high volume situations, flexible circuits also provide cost advantages associated with efficiency of the manufacturing process employed.
Polymeric films, such as those used as substrates in flexible circuits, tend to be good electrical insulators and exhibit a propensity towards the generation of static electrical charges. One particular form of this phenomenon is known as tribocharging in which contact between dissimilar dielectric materials can result in the accumulation of a static charge at the surface of the flexible circuit. Tribocharging may occur even in the normal handling and bending of the flexible circuit during the manufacture of a hard disk drive. Static charges accumulated in this manner can release a sufficient amount of energy to damage the material balance in the thin (10 xc3x85 to 50 xc3x85) MR layer which reduces or destroys the sensing properties of the read head.
U.S. Pat. No. 4,914,551 discloses circuits as provided on flexible dielectric materials. U.S. Pat. No. 4,231,154 discloses a flexible circuit with conductive traces on one or both major surfaces. U.S. Pat. No. 4,480,288 discloses flexible circuits with circuitry on both major surfaces. U.S. Pat. No. 5,401,913 discloses a multilayer flexible circuit including multiple flexible circuits stacked relative to one another and interconnected using metalized through-holes commonly referred to as vias. All of these references disclose various aspects of flexible circuits, but none of them make mention of features designed to enhance static charge dissipative characteristics and to minimize damage due to electrostatic discharge in devices which use these flexible circuits.
U.S. Pat. No. 5,465,186 discloses a magnetorestistive (MR) sensor element in which the conductive leads are shorted together to provide a low resistance conductive path bypassing the MR element and thereby minimizing electrical current through the MR sensing element during discharge of static electrical charge. These leads may be shorted together by soldering or by a twisted conductor pair prior to the assembly of the hard disk drive. This short is subsequently removed prior to placing the MR head into operation in the magnetic storage system. U.S. Pat. No. 5,710,682 discloses an MR head featuring a shorting bar which connects the wire leads in the MR head prior to installation in a disk drive. When properly installed, the MR head is supported by an air bearing or the disk itself to prevent contact between the exposed MR leads and the shorting bar. U.S. Pat. No. 5,761,009 discloses a magnetorestistive read head having a xe2x80x9cparasitic shieldxe2x80x9d providing an alternative path for currents associated with sparkovers, thus preventing such currents from damaging the read head. The parasitic shield is provided in close proximity to a conventional magnetic shield. As charges accumulate on the conventional shield, current will flow to the parasitic shield at a lower potential than would be required for current to flow between the conventional shield and the sensor element. U.S. Pat. No. 6,125,015 discloses a head gimbal assembly with a flex circuit featuring ESD protection, namely a thin seed layer formed on the underside of the substrate. The seed layer may be of nickel (Ni), chromium (Cr) or other material which may be sputter deposited on the back side of the substrate to reduce static charge buildup. All of these references disclose various techniques for limiting the accumulation of electrostatic charge and damage to magnetorestistive (MR) heads for hard disk drives, but none make mention of the use of flexible circuits featuring a strip of conductive polymer to enhance static charge dissipative characteristics and to prevent damage from ESD to hard disk drives.
Conductive polymers, conductive polymer coatings and uses for them are also known. These coatings possess a number of desirable properties including electrical conductivity, optical clarity and scratch resistance. Various formulations are generally applied to glass plates or other substrates by spraying, dipping or spin coating techniques. However, conductive polymers applied using these techniques generally change the dielectric properties of an entire surface of the substrate. These application techniques are generally not suitable for creating a conductive strip on a selective portion of a substrate and, more specifically, have not been applied to enhance the static charge dissipative characteristics of flexible circuits used in magnetorestistive heads for hard disk drives.
Therefore, what is needed is a flexible circuit construction that permits static charges to be controllably and reliably dissipated from a surface of the circuit such that the potential damage from electrostatic discharge (ESD) to electrical components connected to the circuit is reduced.
The present invention is a flexible circuit incorporating electrostatic discharge (ESD) limiting features and designed for use in the fabrication of hard disk drives for computer applications. In accordance with one embodiment of the present invention, a conductive polymer strip is disposed across at least two of the conductive elements of the flexible circuit to reduce the accumulation of static electrical charges and to balance electrical potential across all conductive leads. The conductive polymer strip is preselected to have a high enough resistance to prevent shorting when the circuit is in use while also permitting a desired amount of charge leakage over time when the circuit is not in use. A conductive strip having a resistance ranging from about 104 to about 109 ohms, and preferably about 0.5xc3x97106 to about 1.5xc3x97106 ohms, will not short the circuit when in use and will equalize the static charge across the leads before discharge occurs and further continues to balance the voltages should any discharge occur. By creating this tightly controlled conductive path between the MR leads, it is possible to significantly reduce the likelihood of damage by ESD to an MR head during the assembly of a hard disk drive.